A method of producing compound cast rolls

ABSTRACT

A compound cast roll comprising a shell made of a steel having excellent rolling properties, an arbor having a body portion made of a steel or iron having high toughness and a cylindrical partition member interposed between said shell and said core, said three members being metallurgically connected together into an integral body and subjected to a heat treatment to impart desired properties to said shell and said core.

United States Patent Hachisu et al.

[ 51 May 2,1972

A METHOD OF PRODUCING COMPOUND CAST ROLLS Inventors: Mikio Hachisu;Chikanori Saito; Osamu Sitamura; Yasuo Nambu, all of Katsutashi, JapanAssignee: Hitachi, Ltd., Tokyo, Japan Filed: July 23, 1969 Appl. No.:844,038

Foreign Application Priority Data Jan. 20, 1969 Japan ...44/3432 July26, 1969 Japan ..44/52442 US. Cl ..29/148.4 D, 29/527.6, 29/D1G. 8,

29/D1G. 21, 164/76, 164/95 Int. Cl. ..B2ld 53/12, B2lh 1/12 ..29/148.4D, DIG. 8, 527.6, 29/5275, DIG. 21; 164/95, 96, 98, 76

Field of Search [56] References Cited UNITED STATES PATENTS 517,7474/1894 Harris ..29/148.4 D X 1,011,430 12/1911 Henry ..164/96 288,17611/1883 Harris et a1. ..29/148.4 D

FOREIGN PATENTS OR APPLICATIONS 38/7,608 1963 Japan ..164/96 PrimaryExaminer-John F. Campbell Assistant Examiner-Donald C. Reiley, 111Attorney-Craig, Ant0ne1li& Hill [57] ABSTRACT A compound cast rollcomprising a shell made of a steel having excellent rolling properties,an arbor having a body portion made of a steel or iron having hightoughness and a cylindrical partition member interposed between saidshell and said core, said three members being metallurgically connectedtogether into an integral body and subjected to a heat treatment toimpart desired properties to said shell and said core.

1 1 Claims, 19 Drawing Figures PATENTEDMAY 2 I972 SHEET 10F 6 FIG. 5

INVENTORS MIKID HRC-HISM, cHIKA NoRE SAITo,

05AM SITAMHRA ind YA O NAMUM ATTORNEYS PATENTEBMM 2 I972 3, 659,323SHEET 5 0F 6 FIG. /0 F/G. l3

FIG. F16. /4

INVENTORS MLKL'O HACHISl-( CHI-'KANl/RE SAEFO.

051mm GI-TANHARR and YASuo NAMBM ATTOR NEYfi these properties.

On the other hand, work rolls of one body type used in hot A'MaTnonorPnonucmo tIOMPOUND casrnorts BACKGROUND or THE memos l.Fieldotthelnvention 1 I The present invention relates to compound cast rollsadapted for use inrolling milltacilities. i

2. Description-ofthe Prior Art,

Rolling mill-rolls .are mostlyproduced 'in the form or one bodybypouring a moltenmetal. intoamold of a shape conforming to the shape ofadesired roll. Further, the rolling mill rolls are required to havevarious properties which are variable depending upon the type of roll,but regardless of the type,

they are unexceptionally required to be highly resistive against firecrack, wear, spalling, surface roughening and breakage. In this view,materials of which the rollingmill rolls aremade are desirably of such'achemical composition which will afford these properties.

aforesaid requirements, high alloycast steel maybe named. In general,however, cast steel because of poor castability of a 'highalloy steelmaterial-,one body cast rolls tend to have inter- I As a materialwhichis satisfactory with respectto the 4 Further, in case of back up rollswhich are particularly required to be resistive to wear and spalling, itis known to be essential to increase the hardness as high as possiblebut increasing the hardnessundergoes alimitation because if a high alloysteel is used to increase the hardness of a back up roll of onebody'type, the casting and heat-treating operations are rendereddifficult. In this view, alloy steels consisting of 0.4 to 1.2 percentby weight of carbon, '1 .0 to 3.,0.percentby'weight of chromium, 0.3vto'0.6 percent by weightof molybdenum, 2.0 percent-by weight or less;of nickel and the'remainder of iron, have been used for the productionofj back rolls. Because of such composition of the alloy; steels used.the product back up rolls had a hardness jot not higher than Rs 65 andwere susceptible to wear and spalling. I I f For the foregoing reasonsand in'the light of thefac'tthat the depth of .a roll which is requiredto be resistiye' against fire crack, wear, spalling, surfacerougheningandbreakage is 1.1 to 3 times the available thickness of thewhat largest, in consideration of .the safety factor of the roll," ormore specifically is 30 to 100mm. for the workrolls foruse in a hotstrip mill,

lOOto 250 mm.1 for'the back up rolls'and 100 to 300 mm. for

the ,slabbing'mill rolls andblooming rolls, the production of compoundrolls, including a shell constituting a surface layer of the roll, isalready started in the field of cast iron rolls. As

' this type of rolls,tcast iron compound rolls'which' comprise a weightor more of alloy elements 'and further that it is essentially impossibleto 'use such cast steel forthe production of a practical roll byreasorrof a problem involvedinthe heat treatmenL'The'us'e of high' alloycast steelfor the production of one body type-ro ll -is also undesirablefrom the standpointof economy. i f 1 '1 Under the circumstances, plainsteels and low alloy steels have'come to be usedwhich are satisfactoryin respect of castability and are relatively inexpensive. Itcanno't besaid that-these materials are entirely satisfactory with respect to all7 properties required for the .rolls,-, however. For instance,

slabbing mill. rolls and blooming mill rolls have commonly been producedby a method comprising casting a low alloy steel'consistinggenerally'of0.6 to l.2 percent by weight of carbon, 0.4 to 2.0 percent by weight ofchromium, 0.2 to 0.6

percent byiweight of molybdenum, 04 161. 2 percent by weight, of nickel,0.4 to,:'1.2 percent by weight-of manganese, 0.3 to',l.0 percent; byweight of silicon and the remainder of iron, and subjecting the castroll to a heattreatment. The rolls thus produced have a hardness of j Hs27 to 40.. It has frequently been experienced in the past, however, thata heat crack occurs in the surface of the rolls, upon contact with aheated ingot during rolling operation, and said heat crack provides acause of breakage of the rolls.

' The major properties which are required for the abovementioned.slabbing mill rolls and blooming rolls .are resistances t0 firecrack,wear and breakage, but these properties are incompatible with hardnessandstrength. Therefore, it is almost impossible for a one body type rollsingly to have all of present in the structure of the rolls. provides acause of heat crack which in turn causes a premature roughening of theroll surface. These work rolls ,used in hot rolling are mainly requiredto be resistive to fire crack, wear and surface roughening, but again itis impossible fora work roll of one body type to have all of theseproperties in'the light of its corn 7 position. i i

cast iron compound roll is usually produced by pouring a molten'metalinto a casting mold ofashape conformingtofthe shellmade of a castiron'containing a relatively large amount cast iron, and sleeve fittingrolls or sleeve rolls'which comprise a sleeve and an arbour mechanical Iother, are presently available. I I, v I

A shell-core type compound'roll has not been put in use in which theshall is made of a high alloyca'st steel and the core is made ofplain-cast iron, plain cast steel m]; low alloy steel, however. This isbecause of the followingreason: Namely, a

shapeof the compound roll, allowing themolten metal to remain in saidmold until'a portion of the moltenmetal adjoining the inner surface ofthe mold or a portion which will con- .stitute the'shell of thecompoundroll has been solidified,

thereafter removing the unsolidified metal in the central por-f tion ofsaidmold therefrom to provide for the formation of the core of thecompound roll and finally pouringa core-forming molten metal into theunsolidified shell-forming metal. However, the pouring temperature ofthe high alloy cast steel is so high that it is difficult to adjust themolten high alloy cast steel to a suitable pouringtemperature, andmoreoverthe solidification speed of the high alloy cast steel is so highthat it is ex-v tremely difficult to remove, the central steel to form auniform shell. y v However, a cast iron compound roll, the shell ofwhich is made of a cast iron containing a relativelylarge amount ofalloy elements, is inferior to a compound roll whose shell is portion,of the molten made of high alloy cast steel in respect of toughness, andcannot be used in recent rolling mills which arerequired to be operableunder severe rolling conditions. Hence, the range of use of the castiron compound roll is considerably limited.

7 The term high alloy steel" as used in the description is generalreference to those steels which'contain 0.2 to 2.6 percent by weight ofcarbon'and 3 percent by weight or more in total amount of alloyelements; the term "low alloy. steel to those steels in which the totalamount of alloy elements is 3 percent by weight or smaller; theterrnplain steel to those I steels which contain alloy'elements in only suchan amount as ,that of impurities; and the term plain cast iron to thosecast irons which contain 3.010 3.6 percent by weight of carbon and noalloy element or 3 percent by weight or less alloy elements as required.Y Y

SUMMARY OF THE INVENTION An object of the present invention is toprovide a novel compound cast roll which comprises a shell and a bodyporly cornbined-with each tion of an arbor clearly separated from eachother by a cylindrical partition member.

Another object of the invention is to provide a compound cast roll inwhich the shell is made of a high alloy cast steel.

Still another object of the invention is to provide a compound cast rollin which the shell and the body portion are rigidly combined with eachother.

Still another object of the invention is to provide a compound cast rollin which the rolling characteristic of the roll is uniform substantiallythroughout the width of the shell.

A further object of the invention is to provide a compound cast roll inwhich the shell has properties suitable for rolling, such as resistancesto fire crack, wear, spalling, surface roughening and breakage, whilethe arbor has high toughness.

An additional object of the invention is to provide a novel method ofproducing a compoundcast roll in which the shell is made of a high alloycast steel.

The compound cast roll according to the present invention is composed ofa steel shell having excellent rolling characteristics and a highlytough arbor. The shell and the arbor are metallurgically bonded witheach other through the intermediary of a cylindrical partition member,and said shell is formed such that the hardness thereof is notsubstantially reduced over a portion of its width from the surface ofthe shell at least to a portion adjacent the cylindrical partitionmember.

In the past, it has been believed that a compound roll cannot I be madeof high alloy cast steel but according to the present invention suchcompound roll can beproduced by interposing the cylindrical partitionmember between the shell and the body portion of the arbor, as describedabove. A preferable compound cast roll according to the presentinvention which comprises a shell of high alloy cast steel, is composedof a shell made of a high alloy cast steel which contains 0.2 to 2.6percent by weight of carbon and 3 percent by weight or more in totalamount of alloy elements, and a core made of a member selected from thegroup consisting of low alloy steel, plain cast iron and plain carbonsteel, said shell and said core being metallurgically bonded-with eachother through the intermediary of a cylindrical partition member andsaidshell being formed such that the hardness thereof is not substantiallyreduced over a portion of its width from the surface of the shell atleast to a portion adjacent said cylindrical partition member. 7 1

As stated previously, it is almost impossible to produce a one body rollof high alloy cast steel of 800 mm. or larger in diameter. due to theproperties of said cast steel relative to casting and heat treatment.However, high alloy cast steel has excellent resistance to fire crack,wear, spalling, surface roughening and breakage, and further theseproperties are required for a thickness 1.1 to 3 times the availablethickness of the shell. Therefore, it will be of great practicaladvantage as well as of economical significance if only the shell of theentire roll could be made of high alloy cast steel. Based on this idea,according to the present invention the shell is made of high alloy caststeel and arbor of a member selected from the group consisting of lowalloy steel, plain cast iron and plain carbon steel.

However, if attempt is made to produce such compound roll by the samemethod as that used for the production of the conventional cast ironcompound rolls, i.e., by pouring a molten high alloy cast steel into amold of a shape same as the shape of the product roll, after formationof a solidified layer in a predetermined thickness and then with removalthe central unsolidified metal from the mold pouring an arbor formingmolten metal into the cavity surrounded by said solidified layer, itwould be impossible to obtain a homogeneous shell of uniform thicknessbecause the solidification speed of the high alloy cast steel is veryhigh and removal of the unsolidified metal after formation of thesolidified surface layer would be difficult due to the high viscosity ofsaid cast steel, and further internal shrinkage would result in thecontacting surfaces of the shell and the arbor. According to the presentinvention, therefore, not only is to shell formed of a high alloy caststeel and the body portion of the arbor formed of a member selected fromthe group consisting of low alloy steel, plain cast iron and plaincarbon steel, but also a cylindrical partition member is interposedbetween the shell and the body portion of the arbor.

The partition member is preferably cylindrical in shape to uniformalizethe thickness and rolling properties of the shell. This cylindricalpartition member serves to produce a metallurgical connection betweenthe shell and the body portion of the arbor and to prevent migration ofsaid shell and said arbor. In order for the partition member to servesuch purposes, it must be capable of metallurgical connection withboththe shell and the body portion of the arbor. The present inventorshave found that the degree of metallurgical connection between thepartition member and the shell and arbor depends upon the thickness,composition, properties and surface condition of the partition member.Speaking practically of the properties, the tensile strength of thepartition member may be 30 kg./mm. or greater for said partition memberto withstand a stress imposed thereon. Further, the tensile strength ofthe metallurgical connection must be 10 kg./mm. or greater in the radialdirection of the roll.

Preferably, the proportion of the cross sectional area of the partitionmember to that of the entire-roll is from 2 to 15 percent. As for thecomposition of the partition member, the melt-bonding property of thepartition member is improved as the carbon content increases, butexcessively high carbon content results in melting-away of the partitionmember, allowing inter-mixing of the shell and the core. Therefore,selection of the partition member must be made based on the pouringtemperatures and the pouring times of both the shell and the arbor. Asfor the surface condition, carbon is coated or aluminum is applied tothe surface of the partition member so as to prevent formation of anoxide film thereon. Occasionally, the surface of the partition membermay be subjected to such a treatment as carbonization, nitrification oraluminization, which is effective to improve the bonding property of thepartition member.

It is also an important feature of the compound cast roll according tothe present invention that the hardness of the shell is notsubstantially reduced from the surface thereof to at least a portionadjacent the partitionmember. Such feature is essential to obtain astable rolling performance of the roll over an extended period. Becauseof the presence of the partition member, the solidification of the shellcan be attained adequately and the effect of heat treatment can beextended uniformly throughout the shell. The thickness of the shell mustbe at least as large as the effective thickness of the roll, but inpractice it is desirable that the thickness is 1.1 to 3 times theeffective thickness of the roll, in consideration of the safety factorof the roll. Therefore, the partition wall is preferably located at adepth from the roll surface 1.1 to 3 times, particularly 1.2 to 2.5times, the available thickness of the roll.

In order to afford desired properties to the shell, a high alloy caststeel of which said shell is formed must contain 0.2 to 2.6 percent byweight of carbon and 3 percent by weight or more in total of alloyelements. This is because, if the carbon content is smaller than 0.2percent by weight, a desired strengthv cannot be obtained, whereas ifthe carbon content is larger than 2.6 percent by weight, thewear-resisting property of the shell is degraded to the level of thecast iron. On the other hand, the alloy elements form carbides uponbeing compounded with carbon or are dissolved into the matrix tostrengthen said matrix. They also improve the quenching effect andprovide necessary properties to the shell. As these alloy elements, Ni,Cr, Mo, V, Ti, W, Si, Mn, etc., are usually used and these elements areeffectively combined with each other to afford desired properties to theshell. If the content of these elements is 3 percent or less in total, asatisfactory rolling properties of the roll cannot be obtained as hasbeen experienced with the conventional one body roll, because thehardness of the shell cannot be made uniform throughout the thicknessthereof, due to insufficient quenching effect. On the other hand, if thecontent of these elements is 2.5 percent or more, the castability of thesteel is generally degraded unless the elements used are incorporated inthe steel with particular care. The present invention proposes to use,as a material of the shell, a cast steel based on C-Cr. Cr is used inmany tool steels as a carbide-forming element for promoting thequenching effect and, therefore, has sufficient properties as a rollmaterial. By changing the amount of Cr in the range from 3 to 20 percentwith respective to a carbon content of 0.2 to 2.6 percent, Cr can beapplied to many rolls. Cr is also one of economical elements. The morethe contents of C and Cr are, the more the amount of carbide formed willbe and thus a roll material of excellent wear-resistance can beobtained. However, the toughness of the roll material is degraded on theother hand. For this reason, Ni, Mo, V, Ti, Mn, Si, W, etc. areincorporated in the steel as alloy elements, each in an amount of notmore than 3 percent, whereby a roll material can be obtained which istough and enables the quenching effect to be promoted and further hasexcellent rolling properties.

Namely, Ni and Mn serve to strengthen the matrix and promote thequenching effect; Mo, W and Si serve to improve the mechanicalproperties of the matrix at elevated temperatures and form carbides toimprove the wear-resistance; and V and Ti serve to produce a finestructure of the cast steel to increase the strength thereof. Bysuitably incorporating these elements in a C-Cr base steel, it ispossible to obtain an excellent high alloy shell material.

The properties required for a roll are variable depending upon the typeof the roll since the conditions in which the roll is used are variable.Accordingly, the chemical composition of a high alloy cast steel ofwhich the shell is formed should be adjusted in accordance with the typeof a desired roll, while refraining from making the material cost undulyhigh. In case, for example, of a blooming roll or the like for which abreakageresisting property is required in particular, a high alloy caststeel is used for the formation of the shell which comprises 0.2 to 0.8percent by weight of carbon, 3 to 6 percent by weight of chromium andsubstantially the same amounts of other elements as contained in theconventional roll. In case of a work roll for which a firecrack-resisting property and a surface roughening-resisting propertiesare required in particular, a high alloy cast steel is used whichcomprises 0.8 to 2.0 percent by weight of carbon, 6 to 12 percent byweight of chromium and substantially the same amounts of other elementsas contained in the conventional roll. Further, in case of a roll whichis particularly required to be resistive against wear, a high alloy caststeel is used which comprises 1.5 to 2.6 percent by weight of carbon, to15 percent by weight of chromium and substantially the same amounts ofother elements as contained in the conventional roll.

The partition member must be of such a material which is capable ofproducing a metallurgical connection between the shell and the arbor.Since the partition member is used essentially for the purpose ofpreventing the formation of internal shrinkages in the contactingsurfaces of the shell and the body portion of the arbor, andsimultaneously of preventing the excess migration of the shell-formingmaterial and the arbor forming material, it may be required to be in alength equal to the length of the roll body. The journals of a roll areusually made of the same material as the body portion but since they arerotatably supported by parts of a rolling mill in frictional engagementtherewith, they are susceptible to wear. In order to deal with this,according to the present invention the partition member may be made of amaterial more resistive to wear than the core material and the length ofsuch partition member may be extended to a length equal to the overalllength of the arbor, so that the journals of the roll may be coveredwith the partition member.

It is essential that a portion of the partition member which issubjected to the heat of a shell-forming molten metal at first in thecasting operation, that is, a portion of the partition member located inthe bottom of a casting mold, is not deformed or molten away during thecasting operation. The present inventors have found that it isadvantageous from the standpoint of roll performance to reduce the wallthickness of the partition member from the bottom to top so as toproduce a positive temperature gradience in the solidification of theshell-forming molten metal. In other words, it is necessary to changethe wall thickness of the partition member progressively from one end toanother. Namely, the present inventors have found that when thepartition member is disposed in the casting mold with the thicker wallend thereof located in the bottom of said mold, a portion of thepartition member which is held in contact with the molten metal for thelongest time during casting operation is thicker in wall thickness thanthe other portion, so that said portion of the partition member will notbe molten and the solidification of the molten metal will take placewith a positive temperature gradience. By forming concaves and convexesin the surface of the partition member, a mechanical connection can beproduced simultaneously with the metallurgical connection. Where it istechnically difficult to give a taper to the partition member or theoperation of tapering the partition member requires large labor, aplurality of partition member sections, respectively having differentwall thicknesses, may be arranged longitudinally and welded together. Inthis case, assembly of such partition member sections may be facilitatedby making either the outer or inner diameter thereof same. In order toinsure smooth flow of the shell-forming molten metal, it is preferableto make the inner diameter of the partition member uniform, whileforming steps on the outer surface thereof. The space between the topend of the partition member and the inner surface of the mold should bereduced to minimum, so as to avoid excessive mixing of the shell-formingmolten metal with the arbor-forming molten metal. As stated previously,to subject the partition member to a surface treatment so as to avoidthe formation of an oxide film or to form a carbon film on the surfaceof the partition member is advantageous for facilitating themetallurgical connection among the partition member, the shell and thearbor. The experiments conducted by the present inventors have revealedthat formation of a carbon film (includes graphite film) is mosteffective for this purpose.

The partition member must have a certain mechanical strength per se, toprovide the connection between it, and the shell and the arbor, with amechanical strength sufficient to withstand a stress applied theretoduring rolling operation. In this view, the material of which thepartition member is fonned is preferably selected from the groupconsisting of plain carbon steel, low alloy steel and cast iron whichhave a tensile strength of about 30 kg./mm. or greater. Where it isdesired to constitute the surface layer of the journal of a roll withthe partition member, a high carbon steel excelling in wear resistancemay be used for the formation of the partition member, and in this case,lowering of machinability becomes a problem. Such problem can be solvedby employing the centrifugal casting method, however.

In practice, the compound cast roll according to the present inventionis produced by setting a cylindrical partition member having apredetermined diameter in a casting mold, pouring a shell-forming moltenmetal of steel having excellent rolling properties into an annular spacebetween the casting mold and the partition member and pouring acore-forming melt of steel or iron having high toughness into the hollowof the partition member.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view, partlyin section, showing the appearance of a rolling mill roll according tothe present invention;

FIG. 2 is a photograph showing a cross-sectional view taken along theline 11-11 ofFIG. 1;

FIG. 3 is a microscopic photograph of the cross-section of FIG. 2,showing the structure at the border of the shell and the partitionmember;

FIG. 4 is a microscopic photograph, similar to FIG. 3, showing thestructure at the border of the body portion and the partition member;

FIG. 5 is a perspective view showing in cross-section the constructionof a mold used for practicing the present invention;

FIG. 6 is a transverse cross-sections view of a roll in which thepartition member is extended over the journals thereof;

FIG. 7 is a diagram graphically showing the hardness distributions of aone body cast steel roll and a compound cast roll, relative to thedistance from the roll surface;

FIG. 8 is a diagram graphically showing the forms of chromium carbidesin relation to the amounts of chromium and carbon; and

FIGS. 9 to 19 are cross-sectional views showing the partition membersused in the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1 A compound cast rollfor use as a back up roll, having a diameter of 1,250 mm., a totallength of 4,300 mm., a roll body length of 1,500 mm., a shell thicknessof 200 mm. and a partition member thickness of 25 mm. was produced.

The chemical compositions of the molten metals poured for the formationof the shell and arbor, and the chemical composition of the partitionmember, are shown in Table 1 below:

Before pouring, the shelland core-forming metals of the compositionsshown in Table l were molten in an electric furnace, respectively. Incasting, the shell-forming high alloy cast steel molten at 1,5l5 C. wasfirst poured into the body of the 5 mold through ,the down sprue 7 andthe arbor-forming low alloy steel molten at l,505 C. was concurrentlypoured into the cylindrical member, corresponding to the arbor 2,through the down sprue 8. Upon completion of the pouring, the respectivemetals poured were left to stand in the mold for 7 days for cooling andthen removed from the mold upon completion of cooling. The temperatureof the roll body portion at the time of removal was 1 10 C. Aftercutting the gates, the casting was machined to the dimensions mentionedearlier and thereafter the machined roll was heated to l,050 C. at therate of 25 C./hour, maintained at said temperature for 20 hours, cooledto 400 C., heated again to 830 C., maintained at that temperature for 20hours, allowed to cool to 750 C. and maintained at that temperature for20 hours. In the above 20 process, the roll was heated to and maintainedat 830 C. and

750 C. respectively for the purpose of homogenizing the structures ofthe shell 1 and the core 2, of deforming the carbides into sphericalshape and of removing the casting stress.

Thereafter, the roll was again heated to 980 C. at the rate of 25C./hour, maintained at that temperature for ID hours,

cooled to 400 C. rapidly in a period of 45 minutes and then cooledslowly to 300 C. The roll was again heatedslowly to 550 C. at the rateof 20 C./hour, maintained at that tempera- TABLE 1 Chemical composition(percent by weight) 0 Si Mn P S Ni Cr Mo V Fe 0.52 0.81 0.65 0. 0200.011 1. 42 4. 60 0. 91 0.23 Remainder. 0.62 0 34 0.51 0.015 0.013 0.210.35 Do. 0.21 0.5 Do.

FIG. 1 shows the appearance of the finished roll and the body portion ofthe roll has a cross-sectional structure as shown in FIG. 2 which is atransverse sectional view taken along the line II-II of FIG. 1. FIG. 3is a microscopic photograph showing the structure at the border of theshell and the partition member, while FIG. 4 is a microscopic photographshowing the structure at the border of the partition member and thecore, the magnification being 400 for both photographs. The roll wasproduced by the bottom casting method using the apparatus shown in FIG.5.

Referring to FIG. 2, the shell 1 and the arbor 2 are connected throughtheintermediary of the partition member 3. The partition member 3remains in its original shape even after completion of the casting andserves to prevent migration of the shell-forming molten metal and thecore-forming molten metal. As may be seen in FIGS. 3 and 4, the shelland the partition member, and the arbor and the partition member, aremetallurgically bonded with each other on their borders.

As shown in FIG. 5, the body of a mold, defining a cavity conforming tothe shape of the roll, is provided with a riser gate 4 and the shellportion is composed of a metal mold 5, while the journal-formingportions are composed of sand molds 6. A down sprue 7 is provided forpouring a shell-forming molten metal into the space between the mold andthe partition member, while another down sprue 8 is provided for pouringan arbor-forming molten metal into the hollow of the partition member.

ture for 10 hours, left to stand in the atmosphere for 2 days, againheated and maintained at 550 C. for 10 hours and then subjected tofumace cooling. The rapid cooling from 980 C. and the slow cooling from600 C. are the steps to effect normal quenching and tempering forimparting hardness and toughness to the shell. 1

The hardness of the shell was to in Shore hardness. The tensile strengthof the shell was measured on a sample piece cut away from the shell, tofind that the tensile strength was to kg./mm. The tensile strength ofthe connec tion between the partition member, and the shell and thecore, was about 15 to 25 kg./mm. which is sufficient to withstand therolling conditions. It was also noted that no internal shrinkage norinternal crack was formed at the boundary between the shell and thepartition member and between the core and the partition member, and thatthe compound cast roll thus produced was less inferior to a solid roll.

EXAMPLE 2 A compound cast roll for use as a blooming roll having adiameter of 1,200 mm., a total length of 6,500 mm., a body length of2,400 mm., a shell thickness of 200 mm. and a partition member thicknessof 30 mm. was produced, said partition member extending over thejournals of the roll.

The chemical compositions of the molten metals poured for 65 theformation of the shell and the core, and the chemical composition of thepartition member, are shown in Table 2 below:

TABLE 2 Chemical composition (percent by weight) 0 Si M11 P S Ni Cr Mo VFe 0. 41 0.45 0.70 0, 020 0. 00!) 0. 60 5.15 1.00 0. 30 Remainder. 0.430.38 0.51 0.018 0.010 D0. lnrtItioII member 0.73 0.40 0.53 0.020 0.0100.10 0.20 I)0.

was set in a mold at a location 200 mm. spaced from the inner surface ofthe mold portion corresponding to the body of roll, and then ashell-forming high alloy cast steel, molten at 1,5 20 C., was pouredinto a portion of the mold cavity corresponding to the shell andimmediately thereafter a core-forming carbon steel, molten at 1505 C.,was poured into the hollow of the partition member.

Upon completion of the pouring, the mold was left to stand for 7 dayswith the metals cast therein, to allow the cast roll to cool and thenthe cast roll was removed from the mold. The

temperature of the roll removed from the mold was 100 C. at

the body portion thereof. After cutting the gates, the cast roll wasmachined to the prescribed dimensions and the compound roll thusmachined was heated to 1050 C. at the rate of 25 C./hour, maintained atthat temperature for hours, 2

cooled to 400 C., heated again to 830 C., maintained at that temperaturefor 20 hours, cooled to 770 C., maintained at that temperature for 30hours, again heated to 980 C. at the rate of C./hour, maintained at thattemperature for 10 hours, cooled rapidly to 400 C. in a period of 50minutes and 25 then cooled slowly to 300 C. Thereafter, the compoundroll was slowly heated to 600 C. at the rate of 20 C./hour, maintainedat that temperature for 10 hours, left to stand in the atmosphere for 2days, again heated to 600 C., maintained at that temperature for 10hours, cooled to 98 C. by furnace cooling, removed from the furnace andthen allowed to cool to room temperature, whereby the roll was subjectedto a heat cavity, corresponding to the shell of the roll, by downcasting and successively thereafter a core-forming low alloy steel,molten at l,500 C., was poured into a portion of the cavitycorresponding to the core of the roll.

Upon completion of the pouring, the mold was left to stand for 7 days,with the metals cast therein, and then the cast roll was removed fromthe mold. After cutting the gates, the cast roll was machined to theprescribed dimensions, and then heated to 1,050 C. at the rate of 25C./hour, maintained at 0 thattemperature for 20 hours, cooled to 400 C.,heated again to 850 C., maintained at that temperature for 25 hours, andcooled by furnace cooling, to effect annealing of the roll forhomogenization of the casting structure, deformation of carbides intospherical shape and removal of the casting stress. Then, the roll wasagain heated to 980 C., cooled 620 C. in air, maintained at thattemperature for hours and then cooled by furnace cooling, to effectquenching and tempering, whereby a necessary hardness and toughness wereimparted to the shell.

It was found that the shell and the partition member, and the core andthe partition member, in the thus produced compound cast roll forblooming, had been completely connected with each other metallurgicallyand that the roll was completely free of internal shrinkage or internalcrack. The shell has a bainite structure and the surface hardnessthereof was about 62 in Shore hardness. This hardness is extremely highas compared with the Shore hardness of 27 to 40 of the conventionalblooming rolls. It was also accertained that the compound cast roll thusproduced had excellent resistance against fire crack, wear and breakage.Table 3 below shows the chemical compositions of the shell, the core andthe partition member.

TABLE 3 Chemical composition (percent by Weight) C Si M11 P S Ni Cr Mo VFe Shell 0.48 0. 55 1.20 0.010 0.011 0.68 7.23 0.41 0.21 Remainder.Arbor 0.61 0.36 0.61 0.015 0.000 0.18 0.38 Do. Partitionmember 0.51 0.320.41 0.018 0.010 0.20 0.30 0.15 l)o.

treatment.

In FIG. 6 which shows a vertical cross-sectional view of the roll,reference numeral 8 designates the journal of the roll. As seen, thethickness of the portion of the partition member constituting thesurface layer of the journal differs from the thickness of the remainingportion of the same covering the roll body portion, because the portionof the partition member at the journal was ground after the roll hadbeen cast.

The compound roll thus produced showed a Shore hardness of 5 7 to 62 onthe surface of the shell, a Shore hardness of 40 to 45 at the journalsand a Shore hardness of 26 to 29 at the core. It has been verified thatthe shell and the journals shown sufficiently stable performances as aroll. It was also revealed that the shell and the partition member, andthe core and the partition member were connected completelymetallurgically,

and no internal shrinkages were observed at the connections.

EXAMPLE 3 A compound cast roll adapted for use as a blooming roll andhaving a diameter of 1,200 mm., a total length of 6,000 mm., a roll bodylength of 2,000 mm., a shell thickness of 200 mm., and a partitionmember thickness and length of 25 mm. and 200 mm. respectively, wasproduced by the following process.

Namely, a partition member was set in a mold at a location 200 mm.spaced from the inner surface of the roll body-forming cavity of themold and then a shell-forming high alloy cast steel, molten at l,5l0 C.,was poured into a portion of the EXAMPLE 4 A back up roll adapted foruse in hot rolling and havinga diameter of 1,350 mm., a total length of5,000 mm., a roll body length of 1,700 mm., a shell thickness of 230mm., and a partition member thickness of 25 mm., was produced in thesame manner as in Example 1.

After casting, the mold was left to stand for 9 days, with the castmetals therein, and then the cast roll was removed from the mold bybreaking said mold. After cutting the gates, the cast roll was machinedinto the prescribed dimensions, and then heated to 1,050 C., maintainedat that temperature for 30 hours, cooled to 400 C. in air, heated againto 830 C., maintained at that temperature for 30 hours and cooled byfurnace cooling, whereby annealing of the roll was effected forhomogenization of the casting structure, deformation of carbides intospherical shape and removal of the casting stress.

Then, the roll was again heated to 950 C., cooled in air to 560 C.,maintained at that temperature for hours and then cooled by furnacecooling. Upon examining the compound cast roll thus produced, it wasfound that the shell and the partition member, and the core and thepartition member, had been completely connected with each othermetallurgically, and no internal shrinkages nor internal cracks werefound at the boundaries between the shell and core, and the partitionmember. The surface hardness of the shell was or higher in Shorehardness as desired. The chemical compositions of the shell, the coreand the partition member are shown in Table 4 below:

TABLE 4 Climnioal composition (percent. by weight) Mn l' Ni p (nu V F0 40. 8!! 0, 33 (Mil 0, 020 0. 01 0,68 4, 32 0, 81 0. 10 1021111011001.-Ai'lml'." 0135 0, 38 0.00 0,010 0. 010 0,13 0. 0X 0, 31 D0.

Partitionmomlwi; 0.28

EXAMPLE 5 A compound cast roll adapted for use as a work roll and havinga diameter of 800 mm., a total length of 4,500 mm., a roll body lengthof 2,060 mm. and a shell thickness of 80 mm. was produced by the samecasting process and heat-treating process as in Example 1.

The chemical compositions of the molten metals poured in the mold forthe formation of the shell and the core, and the chemical composition ofthe partition member used, are shown in Table 5 below:

TABLE 5 (lu-mical composition (percent by Weight) 0 Si Mn 1 S Ni (7r MoV Fo hr-ll 0.60 0. 80 0. 50 0.013 0.010 02.0 11.20 0.80 0. 30 Remainder.Arbor 0. 55 0. 40 0. 60 0. 015 0. 00'.) 0. 0. 80 0. l)o. lurtitionmember 0.12 0.30 0.30 0.020 0.013 Do.

The compound cast roll thus produced was completely free 20 Theproperties required for the shell depend largely on of internalshrinkage or internal crack, and the boundaries between the shell andthe partition member and between the core and the partition membershowed a complete metallurgical connection.

REFERRING EXAMPLE 1 A solid cast steel roll having the same diameter,total length and roll body length as those of the roll in Example 1 andcontaining the largest possible amount of alloy elements as allowed bythe castability of the roll material into solid roll, was produced inthe following manner:

Namely, first of all the cast steel, molten at l,500 C., was poured intoa mold having a shape conforming to the shape of the product roll bydown casting and upon completion of the pouring the mold was left tostand for 7 days with the cast steel therein. After removing the castroll from the mold, the gates were cut off and thereafter the roll wassubjected to a heat treatment in the same manner as in Example 4. Thechemical chromium which has a good heat-treatment property and increasesthe hardness of the shell by forming a carbide. in this respect, it isdesirable to use a high alloy cast steel, containing a large proportionof chromium, for the formation of the shell, but on the other hand, theamount of carbon is restricted which is an important element of theshell-forming cast steel having excellent rolling properties. Therefore,the content of chromium is inevitably subjected to a limitation. Thepresent inventors conducted a study on the structure of chromium carbidewith respect to the relative amounts of chromium and carbon, the resultof which in FIG. 8.

The cast steel hitherto used for the production of a solid cast rollvcontains not more than 3 percent by weight of chromium and its structureconsists of a mixture of a and (Fe-Cr) In a region defined by a lineconnecting a point v representing 0.06 percent by weight of carbon and2.0 percent .by weight of chromium with a point representing 2.6 percentby weight of carbon and 4.5 percent by weight of chromium, and a lineconnecting said first point with a point representing composition of thecast steel used is shown in Table 6 below:

TABLE 6 Chemical composition (percent by weight) 0 Si Mn P s Ni of M0 FeT One body roll 0.50 0.50 0.50 0.014 0. 010 0.50 2.20 0.30 Remainder.

FIG. 7 is a graph showing the hardness distributions of the compoundcast roll obtained in Example 1 and the solid cast steel roll obtainedin Control Example 1. The hardness distribution was obtained bymeasuring the hardness at various spots and the distances of therespective spots from the roll surface. As seen, the hardnessdistribution of each roll has a certain width because since, even whenthe distance is the same, the hardness was not the same at differentspots of measurement, the measurements were taken 8 times in average ata spot and the maximum and the minimum values of hardness at therespective spots were plotted. From the chart of FIG. 7, it will be seenthat the surface hardness of the solid cast steel roll is about 60 inShore hardness, which is slightly low for the roll to be used as a backup roll but is sufficiently high for the roll to be used as a bloomingroll and a work roll. This solid cast steel roll has a tendency that thehardness decreases rapidly with the radial distance and such tendencybecomes particularly apparent from a distance of 70 mm. and onwards fromthe roll surface. Such a sharp decrease in hardness at a short distancefrom the roll surface is objectionable from the standpoint ofperformance of the roll for the following reason: Namely, supposing thatthe effective thickness of the roll is, say 100 mm., there should not bea substantial hardness decrease in said thickness because, if otherwise,the properties of the roll would vary as the rolling operation proceedsand the roll would be cracked or rapidly worn off before the effectivethickness is exhausted even if the rolling operation is performed underthe same conditions.

2.6 percent by weight of carbon and 22.0 percent by weight of chromium,the structure of the cast steel consists of a mixture of a, (Fe-Cr) Cand (Cr-Fe) C and the amount of carbide formed is more than in the casewhen the chromium content is not more than 3 percent by weight. Thissubstantiates the fact that the hardness of the cast steel increaseswith the amount of chromium.

Now, in a region wherein the amount of chromium is more thanabove-mentioned region and which is defined by a line connecting a pointrepresenting 0.06 percent by weight of carbon and 2.5 percent by weightof chromium with a point representing 2.6 percent by weight ofcarbon and22.0 percent by weight of chromium, a line connecting said first pointwith a point representing 0.02 percent by weight of carbon and 10.0percent by weight of chromium, a line connecting said second point witha point representing 2.6 percent by weight of carbon and 30.0 percent byweight of chromium, the structure of the cast steel consists of amixture of a and (Cr-Fe) C and the amount of chromium carbide formedfurther increases. When the amount of chromium-is further increased,(Cr-Fe).,b83 comes to be formed, rendering the cast steel extremelyfragile. In addition, the castability of the steel is degraded and thetoughness of the same is reduced, with the result that the steel cannotbe used for the production of a roll.

The present inventors conducted a further study to determined a regionwherein the amount of chromium carbide can be increased withoutjeopardizing the castability of the steel, and found that the onedefined by a line connecting a point A representing 0.2 percent byweight of carbon and 3.0 percent by weight of chromium with a point Brepresenting 0.2 percent by weight of carbon and 8.0 percent by weightof chromium, a line connecting said point B with a point C representing1.6 percent by weight of carbon and 20.0 percent by weight of chromium,a line connecting said point C with a point D representing 2.6 percentby weight of carbon and 20.0 percent by weight of chromium, a lineconnecting said point D with a point E representing 2.6 percent byweight of carbon and 4.2 percent by weight of chromium, and a lineconnecting said point E with said point A, is most preferable. Namely,in case of a composition falling in a region above the line BCD in FIG.8, not only does Cr fonn a carbide with C but also a large amount of Cris dissolved in the matrix, rendering the matrix itself fragile.Further, in case of a composition falling in a region on the right sideof the line DE, the amount of carbide formed so large that the steelbecomes fragile and cannot be used for the production of a roll. For thereasons set out above, the composition range of the steel which can beused for roll is naturally limited.

The partition member used in the present invention may be provided inthe shapes shown in FIGS. 9 to 16. A partition member shown in FIG. 9 isin the shape of a jointless cylinder of uniform wall thickness and canbe produced simply as by centrifugal casting method. A partition membershown in FIG. 10 has a cylindrical inner surface of uniform diameter anda tapered outer surface, and can also be produced simply as bycentrifugal casting method. A partition member shown in FIG. 11 has acylindrical inner and outer surfaces of uniform diameter and consists ofa plurality of sections connected with each other by welding. Apartition member shown in FIG. 12 has welded joints, not in alongitudinal direction but in a circumferential direction thereof. Apartition member shown in FIG. 13 consists of a plurality of sectionsconnected together into a unitary piece by welding, which sections areuniform in inner diameter but difierent in outer diameter. A partitionmember shown in FIG. 14 consists of a plurality of sections connectedtogether into a unitary piece by welding, which sections aresubstantially uniform in wall thickness but different in inner and outerdiameters. A partition member shown in FIG. 15 consists of a pluralityof sections connected with each other by welding into a unitary piece,which sections have parallel inner and outer surfaces but are differentin inner and outer diameters, the outer diameter becoming larger as theinner diameter becomes smaller. A partition member shown in FIG. 16consists of a plurality of sections connected together into a unitarypiece by welding, which sections have a uniform outer diameter butdifferent inner diameters. A partition member shown in FIG. 17 has aplurality of peripheral grooves formed in the outer surface thereof soas to produce a mechanical connection in addition to a metallurgicalconnection. As a modification, a partition member having such groovesformed in the inner surface or in both the inner and outer surfaces mayalso effectively be used.

The principle of the present invention can of course be applied to aroll, such as a die steel roll, which has a concavoconvex surface. Inthis case, the partition member may simply be incorporated in the rollin a manner as shown in FIG. 18, without excerting a special ingenuity,as the effective roll thickness is experientially known to be severalmillimeters from the roll surface. FIG. 19 shows a partition memberhaving a flared top end which is effective for preventing excessivemigration of the shell-forming molten metal and the coreforrning moltenmetal into each other through a space between the partition member andthe mold surface.

We claim:

1. A method of producing a compound cast roll with an outer metal shellhaving excellent rolling properties and a metal core having excellenttoughness, comprising the steps of placing a cylindrical partitionmember having a predetermined diameter in a casting mold, the length ofthe partition member being approximately equal to that of said shell;pouring a shell-forming molten metal into an annular space between saidpartition member and said mold; pouring a coreforming molten metal intoan interior space formed by said partition member; forming the resultantcasting into a desired roll shape by machining and subjecting the formedroll to a heat-treatment operation for imparting the desired rollingproperties to the shell and the desired toughness to the core, saidpartition member having a wall thickness that is reduced from the lowerend adjacent the bottom of the mold toward the upper end adjacent thetop of the mold whereby solidification of the molten shell-forming andcore-forming metals takes place under a positive temperature gradient.

2. A method of producing a compound cast roll as defined in claim 1, inwhich said partition member is set in the mold in such a manner that theshell formed will have a thickness 1.1 to 3 times the availablethickness of the roll.

3. A method of producing a compound cast roll as defined in claim 1, inwhich said partition member has a flared portion at its upper end toreduce the space between said partition member and said mold, wherebyexcessive migration of said shell-forming molten metal and saidcore-forming molten metal into each other is prevented.

4. A method of producing a compound cast roll as defined in claim I, inwhich said partition member has a carbon film formed on the surfacethereof.

5. A method of producing a compound cast roll as defined in claim 1, inwhich said partition member consists of a plurality of sectionsconnected into a unitary structure by welding.

6. A method of producing a compound cast roll as defined in claim I, inwhich said partition member is a centrifugally cast tube.

7. A method of producing a compound cast roll as defined in claim 1, inwhich the wall thickness of said partition member at the lower portionthereof is so selected that said lower portion of the partition membermay not be deformed or melted away during the steps of pouring theshell-forming and core-forming metals into said mold.

8. A method of producing a compound cast roll as defined in claim 1, inwhich the predetermined diameter and the wall thickness are so selectedthat the proportion of the cross-sectional area of said partition memberto the cross-sectional area of the entire compound cast roll is 2 to 15percent.

9. A method of producing a compound cast roll as defined in claim I, inwhich said partition member prevents migration of the shell-formingmolten metal into the core-forming molten metal and provides ametallurgical bond between said mol ten metals.

10. A method of producing a compound cast roll as defined in claim 1, inwhich said shell-forming cast steel comprises carbon, chromium, otheralloy elements as required and iron, the amounts of carbon and chromiumfalling within a range which is defined, in an orthogonal coordinatesystem having the ordinate axis scaled by the amount of chromium and theabscissa axis scaled by the amount of carbon, by a line connecting apoint A representing 0.2 percent by weight of carbon and 3.0percent byweight of chromium with a point B representing 0.2 percent by weight ofcarbon and 8.0 percent by weight of chromium, a line connecting point Bwith a point C representing 1.6 percent by weight of carbon and 20.0percent by weight of chromium, a line connecting point C with a point Drepresenting 2.6 percent by weight of carbon and 20.0 percent by weightof chromium, a line connecting point D with a point E representing 2.6percent by weight of carbon and 4.2 percent by weight of chromium, and aline connecting point E with point A, said core-forming metal being ametal selected from the group consisting of low alloy steel, plain castiron and plain carbon steel.

11. A method of producing a compound cast roll as defined in claim 1, inwhich said heat-treatment operation includes annealing of the roll forhomogenization of the structure, deformation of carbides into sphericalshape and removal of the casting stresses.

2. A method of producing a compound cast roll as defined in claim 1, inwhich said partition member is set in the mold in such a manner that theshell formed will have a thickness 1.1 to 3 times the availablethickness of the roll.
 3. A method of producing a compound cast roll asdefined in claim 1, in which said partition member has a flared portionat its upper end to reduce the space between said partition member andsaid mold, whereby excessive migration of said shell-forming moltenmetal and said core-forming molten metal into each other is prevented.4. A method of producing a compound cast roll as defined in claim 1, inwhich said partition member has a carbon film formed on the surfacethereof.
 5. A method of producing a compound cast roll as defined inclaim 1, in which said partition member consists of a plurality ofsections connected into a unitary structure by welding.
 6. A method ofproducing a compound cast roll as defined in claim 1, in which saidpartition member is a centrifugally cast tube.
 7. A method of producinga compound cast roll as defined in claim 1, in which the wall thicknessof said partition member at the lower portion thereof is so selectedthat said lower portion of the partition member may not be deformed ormelted away during the steps of pouring the shell-forming andcore-forming metals into said mold.
 8. A method of producing a compoundcast roll as defined in claim 1, in which the predetermined diameter andthe wall thickness are so selected that the proportion of thecross-sectional area of said partition member to the cross-sectionalarea of the entire compound cast roll is 2 to 15 percent.
 9. A method ofproducing a compound cast roll as defined in claim 1, in which saidpartition member prevents migration of the shell-forming molten metalinto the core-forming molten metal and provides a metallurgical bondbetween said molten metals.
 10. A method of producing a compound castroll as defined in claim 1, in which said shell-forming cast steelcomprises carbon, chromium, other alloy elements as required and iron,the amounts of carbon and chromium falling within a range which isdefined, in an orthogonal coordinate system having the ordinate axisscaled by the amount of chromium and the abscissa axis scaled by theamount of carbon, by a line connecting a point A representing 0.2percent by weight of carbon and 3.0 percent by weight of chromium with apoint B representing 0.2 percent by weight of carbon and 8.0 percent byweight of chromium, a line connecting point B with a point Crepresenting 1.6 percent by weight of carbon and 20.0 percent by weightof chromium, a line connecting point C with a point D representing 2.6percent by weight of carbon and 20.0 peRcent by weight of chromium, aline connecting point D with a point E representing 2.6 percent byweight of carbon and 4.2 percent by weight of chromium, and a lineconnecting point E with point A, said core-forming metal being a metalselected from the group consisting of low alloy steel, plain cast ironand plain carbon steel.
 11. A method of producing a compound cast rollas defined in claim 1, in which said heat-treatment operation includesannealing of the roll for homogenization of the structure, deformationof carbides into spherical shape and removal of the casting stresses.